Protection switching in optical networks enables redundancy and resiliency to fiber cuts or other failures. At Layer 1, e.g., Time Division Multiplexing (TDM) such as Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH), etc., protection switching occurs in the electrical domain (Layer 1) via synchronous signaling. Specifically, there are dedicated bytes in the frame overhead assigned for the purpose of signaling local switch status (switch state, status of working and protection lines, scheduled switch events, etc.) to a remote end. This provides the infrastructure for a bidirectional switching capability that keeps duplex traffic flows on the same path in electrical domain. However, such synchronization framework in the optical domain does not exist. At Layer 0, e.g., optical, to protect fiber spans against line cuts, an Optical Protection Switch (OPS) is used in some configurations to get fast traffic restoration. An OPS typically contains an optical switch in the receiving direction that measures total power in both ports and switches from one port to another port based on loss of light (LOL) detected on that port. To protect fiber spans, OPS switch is placed in the receiving (RX) direction of the fiber spans, while the transmitting (TX) power to the fiber span is broadcasted to protect fiber paths using a power splitter on the OPS
It is important to maintain synchronization in Layer 0 optical protection switching at OPSs at both ends of a link. In an optical network deployment, Layer 0 traffic is usually bi-directional. From a fiber path protection perspective, it is important to ensure that traffic in both directions experiences similar path losses and network elements in-between. This ensures that optical channels running bi-directionally have a similar traffic experience for optical performances in terms of Optical Signal-to-Noise Ratio (OSNR), non-linear penalties, Bit Error Rate (BER), latencies, and available margin. However, with the conventional OPS protection scheme, that cannot be ensured as following a fiber fault, optical channels in both directions can experience performance deltas for an extended period of time until the switch at the non-faulted end is manually switched.
Additionally, in such conventional protection schemes, protected fiber paths (or spans) are designed to be maximally diverse so that with a fault in one path, the other path can be operational with a fast optical switch. Following a fault such as a fiber cut in one path, when trucks are rolled out to re-splice the fibers, both the transmit (TX) and receive (RX) direction fiber-pairs are put into a maintenance state so that re-splicing can be done maintaining laser safety standards. With the conventional protection schemes, before such maintenance procedures, a protection switch at the non-faulted end has to be manually switched that triggers an unnecessary disruption in traffic. In other words, with the conventional protection schemes, physical efforts are required to ensure both fiber paths are identical in nature in terms of span loss, fiber type, physical deployment (e.g., aerial vs. ground fibers), while each fiber path is kept limited to a single span to minimize the path to path characteristics variations. In a real field deployment, with diversified fiber paths used in such OPS protection schemes, maintaining these constraints between two sets of fiber pairs is difficult to achieve.
A conventional approach for synchronization includes introducing an optical shutter in the TX direction. When the OPS switches on the RX port, it can also trigger a shutter off and on in the TX direction, that in turn, triggers a loss of light on the other end and hence, triggers a switch on the other end too. However, it is a more expensive solution as the shutter mechanism requires additional optical components, space, processing, and control. Moreover, this approach requires special handling on how long the shutter needs to be toggled between OFF and ON so that the switch on the other end sees the required loss of light for certain duration to move from one RX port to another. In addition, in some cases, due to enough Amplified Spontaneous Emission (ASE) loading on the path, the other end may not see the necessary loss of light to trigger the switch. Another drawback of using shutter/Variable Optical Attenuators (VOAs) on the OPS is from a reliability point of view. The OPS is a single point of failure for the services traversing it, and therefore it is advantageous to minimize the part count (and especially electrically active parts) for a low probability of failure. Due to this uncertainty, and due to the high dependency between pre-planned ASE estimation and actual network deployment, the expensive TX shutter based OPS hardware solution is not popular in field deployment.